Voice over IP basics (1): The H.323 protocol

Introduction:

Over the next few years the use of
bandwidth will increase and the industry is looking for a solution for the problems,
which will arise by the growing use of bandwidth. The best solution so far is
ATM (asynchronous transfer mode) the switching fabric designed to handle voice,
data and video. This is a good step on the way, but there will be still problems
with network congestion and packet loss. Further on the industry must also tackle
the problems of network reliability and sound quality. This can be done by the
gradual adoption of standards. These standards will focus on three central elements;
the audio codec format, transport protocols and directory services.

The H.323 protocol:

Over the next few years the use of bandwidth will increase and the industry is looking for a solution for the problems, which will arise by the growing use of bandwidth. The best solution so far is ATM (asynchronous transfer mode) the switching fabric designed to handle voice, data and video.

In May 1996, the International Telecommunications
Union (ITU) ratified the H.323 specifications (an adaptation of H.320), which
defines how voice, data and video traffic will be transported over IP based
local area network. H.323 is a set of recommendations and is based on the transport
protocol RTP. The RTP is a new protocol layer for real-time applications. The
limitation of RTP is that the protocol has no mechanisms for ensuring the on-time
delivery of traffic signals or for recovering lost packets. RTP also does not
address the so-called Quality of Service (QoS) issue related to guaranteed bandwidth
availability for specific applications. At the moment there is a draft signaling-protocol
standard that can strengthen the Internet ability to handle real-time traffic
reliably. This makes it possible to setup dedicated end-to-end transport paths
for specific sessions much like the circuit-switched PSTN does. This protocol
is called the resource reservation protocol (RSVP)

H.323 specifies five phases for call
completion:

Call Initiation

Endpoint capability exchanged

Establish the call

Call services (such as bandwidth
utilization)

Call termination.

The H.323 standard describes the roles
that must be played for successful interoperability of the gateway, terminal
devices gatekeeper and multipoint control unit. While H.323 has been widely
accepted as the standard protocol for voice-over-IP networking, another protocol
called Session Initiation Protocol, or SIP, has been gaining popularity as an
alternative. SIP, a client-server protocol, was originally created to control
multimedia calls over the internet as is similar to HTTP.